The skin element has an inner surface facing the interior of the aircraft fuselage structure, and an outer surface. The support structure is connected to the inner surface of the skin element and may comprise a plurality of support elements, such as frame elements or stringer elements. The door opening is provided in the skin element and has an essentially rectangular shape, wherein the corners might be rounded. The rectangular shape relates to a side view on the aircraft fuselage structure where the door opening is visible. Of course, when viewed in the longitudinal direction of the aircraft fuselage structure, the skin element and the support structure follows a curved outline along the circumference of the aircraft fuselage structure.
The support structure comprises an upper longitudinal element provided at an upper longitudinal edge of the door opening, a lower longitudinal element provided at a lower longitudinal edge of the door opening opposite the upper longitudinal edge, a first circumferential element provided at a first circumferential edge of the door opening at a first side with respect to the door opening, and a second circumferential element provided at a second circumferential edge of the door opening opposite the first circumferential edge, i.e. at a second side with respect to the door opening opposite the first side. The longitudinal and circumferential directions are preferably perpendicular with respect to one another and refer to a longitudinal axis of the aircraft fuselage structure or the associated aircraft. The first side may preferably be directed to the front and the second side may preferably be directed to the rear of the aircraft fuselage structure.
Such aircraft fuselage structures are commonly used in the art, wherein the support structure comprises a plurality of frame elements which extend in the circumferential direction of the aircraft fuselage structure, i.e. in parallel to the first and second circumferential elements at the door opening, and a plurality of stringer elements extending in the longitudinal direction of the aircraft fuselage structure, i.e. in parallel to the upper and lower longitudinal elements at the door opening. Such a support structure, comprising a uniform grid of frame elements and stringer elements, wherein the stringer elements extend in parallel to one another and perpendicularly to the frame elements, has been established for its reliability, for its simple manufacturing and mounting, as well as for its good predictability of load cases and failure.
However, the common arrangement of frame and stringer elements does not reflect the location of the actual load paths along the aircraft fuselage structure. Numerical calculations of a variety of load cases have shown that the main load paths along the aircraft fuselage structure do not extend mainly in the longitudinal and in the circumferential directions, but are rather skewed or angled by a certain angle between these both directions, thereby forming a grit or web along the aircraft fuselage structure forming a plurality of triangles instead of only rectangles as done by the common frame and stringer elements. In order to optimize the aircraft fuselage structure for a minimum weight it would be desirable to rearrange the support structure such that the support elements, i.e. the frame and stringer elements, extend along the main load paths instead of in the longitudinal and circumferential directions only.
Yet, such an arrangement of the support structure along the main load paths has not been practically implemented yet, since manufacture and mounting of such an aircraft fuselage structure would be extremely complex and expensive with the common techniques of assembling the aircraft fuselage structure from a plurality of separate parts, such as skin, stringer, frame, which have to be joined e.g. by riveting or bonding. However, with the recent progress in the ALM technology such aircraft fuselage structures including a main load path oriented support structure appear more realistic, as the complex support structure could be simply printed integrally together with the skin element.
While the load paths in some parts of the aircraft fuselage structure may change depending on the respective aircraft geometry, the present disclosure focuses on the surround structure of the door opening, where the load paths remain substantially the same irrespective of the aircraft model, and a complex superposition of main load paths requires particular consideration.